An inert U.S. Hellfire missile sent to Europe for training purposes was wrongly shipped from there to Cuba in 2014, said people familiar with the matter, a loss of sensitive military technology that ranks among the worst-known incidents of its kind.

The unintended delivery of the missile to Cuba has confounded investigators and experts who work in a regulatory system designed to prevent precisely such equipment from falling into the wrong hands, said those familiar with the matter.

For more than a year, amid a historic thawing of relations between the U.S. and Cuba, American authorities have tried to get the Cuban government to return the missile, said people familiar with the matter. At the same time, federal investigators have been tracing the paper trail of the wayward Hellfire to determine if its arrival in Cuba was the work of criminals or spies, or the result of a series of blunders, these people said.